Visions of Coleus is a short story that happens between the first and second season of 'That time I reincarnated as a slime'. It's not crucial to the main story but it does give a bit more information on some elusive characters and explore another nation so it is worth watching in the main series. The story is about Rimuru going to another nation to attempt to gather information about a possible civil war. The story isn't super impressive or anything but it's fun to watch regardless. The characters are mainly just rimuru and and locally relevant characters with actual significant characters occasionally coming up.The characters seem just fine, it's just three episodes and enough information is given to understand the characters but not much more.

The animation, voice acting and sound are all fine and are good enough for the story.

Overall I'd recommend watching this since it does add to both the worldbuilding and characters in the slime world. It's probably best to watch it between the first and second season but watching it afterward doesn't matter much either.